Catalytic processes for the conversion of hydrocarbons are well known and extensively used. Invariably the catalysts used in these processes become deactivated for one or more reasons. Some of these reasons include the accumulation of coke deposits, the accumulation of nonmetallic poisons such as sulfur and nitrogen, the accumulation of metallic poisons such as iron and lead, the agglomeration of catalytically-active metals such as platinum and palladium, and the degradation of the catalyst support such as pore enlargement and phase transformation. Depending on the nature of the catalyst and the kind and degree of deactivation, the catalyst may become deactivated permanently. Catalyst that is neither usable nor regenerable in situ must be removed from the process and replaced with fresh or regenerated catalyst.
One method of replacing catalyst in a process is to stop the normal operation of the process, unload the spent catalyst, load fresh catalyst, and then resume normal operation of the process. Although straightforward, this method requires a period of time when the process is not normally operating. This period of time, called downtime, is lost production time that can be a significant expense in commercial hydrocarbon conversion operations.
A method that replaces the catalyst in a hydrocarbon conversion process should have several objectives. First, it should eliminate downtime, thereby making the process more profitable. Second, it should purge the fresh catalyst prior to loading to remove contaminants that would affect adversely the performance of the process. Third, it should not require a reduction in the normal or design rate of catalyst movement through the process. And, fourth, it should allow the process control systems that ensure reliability during normal operation to continue in operation during the catalyst replacement.
The present invention achieves these objectives in hydrocarbon conversion processes that employ a moving bed of catalyst that is transferred between two or more zones. The present invention is particularly useful for those processes that employ zones containing fluids that are inhibited from communication, such as a process employing a reaction zone containing hydrocarbons and a regeneration zone containing oxygen. If the two zones are connected by a pipeline, communication between the two zones can be inhibited by dosing valves in the pipeline, but this also stops the flow of catalyst between the zones. Besides, valves that dose in pipelines containing flowing catalyst are generally maintenance problems, because of leakage due to wear from catalyst particles and malfunction due to high temperatures.
But neither dosing valves nor stopping catalyst flow are requirements for inhibiting communication between two zones. Introducing an inert fluid into a purge, or buffer, zone between the zones can accomplish this goal, even while catalyst is flowing between the zones. The inert fluid is generally added into a purge zone at a pressure higher than the pressure of either zone. Beginning at the point of introduction of the inert fluid, two portions of the inert fluid flow in opposite directions--one portion toward one zone and the other toward the other zone. Therefore, relative to the flow of catalyst, one portion of the inert fluid flows countercurrently whereas the other portion flows cocurrently. The two portions of the inert fluid effectively purge the catalyst between the two zones and inhibit the communication of the two fluids.
In moving bed processes that employ a purge zone like that just described, the present invention uses the existing purge zone vessel and its associated process control instrumentation to replace the catalyst. Therefore, in addition to achieving the objectives described above, the present invention maximizes the use of existing equipment and minimizes the need for additional equipment that would sit idle except during catalyst replacement. For these reasons, this invention is a simple, efficient, and cost-effective method of replacing catalyst in moving bed processes.